1. Field of the Invention.
The present invention relates to heterostructure semiconductor devices. In particular, the present invention relates to heterostructure planar photodetectors and to high-speed heterostructure integrated circuits which incorporate a planar photodetector.
2. Description of the Prior Art.
The merging of unipolar and bipolar devices has a great potential for increasing speed of integrated circuits. This approach has been proven very fruitful in silicon electronics (represented primarily by BICMOC technology). See for example, T. Y. Chiu, G. M. Chin, M. Y. Lau, R. C. Hanson, M. D. Morris, K. F. Lee, A. M. Voshchenkov, R. G. Schwarts, V. D. Archer and S. F. Finegan, "A High-Speed Super Self-Aligned Bipolar Technology", IEDM Technical Digest, pp. 24-27 (1987). It is even more appealing for compound semiconductor technology where the combination of low power complementary logic and excellent driving capabilities of Heterojunction Bipolar Transistors can allow to increase both speed and integration scale. See for example, N. C. Cirillo, Jr., M. Shur, P. J. Vold, J. K. Abrokwah, R. R. Daniels, and O. N. Tufte, "Realization of n-channel and p-channel High Mobility (Al,Ga)As-GaAs Heterostructure Insulated Gate FETs on a Planar Wafer Surface,"IEEE Electron Device Letters, EDL-6, pp 645-647 (1985). Another important advantage is a possibility to use bipolar devices as sensors and light sources, integrated with field effect transistor and bipolar transistor electronics. Recently the first monolithic integration of a Modulation-Doped Field Effect Transistor (MODFET) and Heterojunction Bipolar Transistor was described by T. Usagawa, S. Goto, T. Mishima, M. Yamne, M. Kobayshi, K. Kawata, and S. Takanashi in IEDM Technical Digest, pp. 78-81 (1987). The principle of a Heterojunction Bipolar Transistor used in that integrated device is similar to that of a Bipolar Inversion Channel Field-Effect Transistor (BICFET). See G. W. Taylor and J. G. Simmons, "A Bipolar Inversion Channel Field-Effect Transistor (BICFET)--A New Field-Effect Solid State Device: Theory and Structures,"IEEE Trans. Electron Devices, ED-32, p2345 (1985).
Difficulty is encountered in integrating planar integrated circuits with conventional optical detectors that require a wide depletion region (typically several tens of microns) in order to absorb all the incident light. Typical detectors are volume, rather than planar devices. Recently a planar-type Heterojunction Field Effect Transistor Photodetector (HFETPD) has been proposed which eliminates the requirement of a wide depletion region. See G. W. Taylor and J. G. Simmons, "High Speed Integrated Heterojunction Field Effect Transistor Photodetector: A Gated Photodetector," Appl. Phys. Lett., 50, No. 24, pp 1754-1756.
The new approach utilized in HFETPD, (and before that, in superlattice detectors proposed by S. Luryi, T. P. Pearsall, H. Temkin, and J. C. Bean, "Waveguide Infrared Photodetectors on a Silicon Chip,"IEEE Electron Device Letters, EDL-8, No. 2, pp 104-106 (1986)) is to have the incident light propagation perpendicular to the carrier flow so that the device has both high speed determined by the transit time in a narrow surface region, and efficient light absorption determined by the gate width.
MODFET integrated circuits have become the fastest solid state technology. See for example, N. J. Shah, S. S. Pei, C. W. Tu, and R. C. Tiberio, "Gate-Length Dependence of the Speed of SSI Circuits Using Submicron Selectively Doped Heterostructure Transistor Technology,"IEEE Trans. Electron Devices, ED-31, p 543 (1986) and N. C. Cirillo, Jr., J. K. Abrokwah, and M. Shur, "A Self-Aligned Process for IC's Based on Modulation-Doped (Al,Ga)As/GaAs FET's," EEE Trans. Electron Devices, ED-31, p. 1963 (1984). Creating MODFET-based integrated optoelectronic circuits is of the utmost importance.